1. Field of the Invention
The invention relates to fluid delivery systems. More particularly, the present invention relates to detecting air and other agents in a fluid delivery system infusing fluid to a patient.
2. Description of Related Art
There are a variety of situations where fluid is infused to a patient. Applications of fluid delivery systems include (but are by no means limited to) intravenous infusion, intra-arterial infusion, infusion of enteral solutions, infusion of medication to the epidural space, and diagnostic infusion to determine vascular characteristics of the arterial, urinary, lymphatic, or cerebrospinal systems.
Fluid delivery systems for infusing fluid to a patient typically include a supply of the fluid to be administered, an infusion needle or cannula, an administration set connecting the fluid supply to the cannula, and a flow control device, such as a positive displacement infusion pump. The administration set typically comprises a length of flexible tubing. The cannula is mounted at the distal end of the flexible tubing for insertion into a patient""s blood vessel or other body location to deliver the fluid infusate to the patient.
During an infusion procedure, various agents, the most typical of which is air, can be introduced into the fluid delivery system by a number of events, including the fluid supply becoming drained of fluid. Because introducing excessive air into the patient""s blood system may create complications, it is desirable to detect the introduction of air into the fluid delivery system before substantial amounts of air are introduced into the patient. When substantial amounts of air are detected in the fluid delivery system, fluid delivery can be terminated until a health care provider can correct the underlying problem, such as by refilling or replacing the fluid supply.
Sometimes, a temporary event, such as the accumulation of small quantities of air from outgassing of air suspended in the solution, may cause a very few small air bubbles to enter the system. Where the amount of air is quite small, the patient may be able to safely absorb the small air amounts, so that stopping operation of the pump is unnecessary. Thus, it is desirable to not only detect the air in the fluid delivery system, but also to evaluate the amount of air present.
One technique for determining the amount of air in a fluid delivery system, such as a length of intravenous tubing, is through the use of sensors such as light or ultrasonic sensors. In such a technique, electromagnetic energy, such as light, or sound energy, such as an ultrasonic pulse, is passed through the intravenous tubing, and the sensor monitors variations in the received energy. Because air generally transmits light and/or sound energy in a different fashion than do intravenous fluid solutions, due to different transmission properties such as absorption and/or refractivity, monitoring variations in the light""s or sound""s ability to pass through the solution can give a generally accurate determination that air exists in the fluid line.
A more difficult problem is determining just how much air is in the fluid line, and how much will be delivered to the patient. For example, at a particular point in time, a sensor looking at just a very short section of the tubing may see only air in the line, with no intravenous fluid solution present. This may be the result of the fluid supply being entirely empty, in which case the fluid delivery system should be shut down. However, a single small air bubble may also cause the same sensor reading, and shutting down the fluid delivery system on account of a single air bubble may be inappropriate.
A small amount of air may be of no consequence where no significant amounts of air are in the delivery system either upstream or downstream of the sensor section. Where the small amount of air is part of a continuous stream of small air bubbles in the tubing, however, the sum of the small bubbles may amount to a significant amount of air, so that the fluid supply system should be shut off pending correction of the underlying problem.
A method of accounting for the limitations of monitoring just a short section of the tubing is to install several sensors along the length of the tubing, thereby monitoring a much longer section of tubing. The addition of multiple sensors and their associated electronics can, however, substantially add to the cost and complexity of the fluid delivery system. Moreover, such use of multiple sensors may still not accurately determine the amount of air in the line over long periods of time or as large volumes of fluid pass therethrough.
A further method is to keep a running total of the air that passes through the tubing section. When the total air reaches a certain threshold, the fluid delivery system can be shut down to await correction of the underlying problem by appropriate personnel. Such a simple running total may not, however, adequately reflect the actual ability of the patient""s system to safely absorb air.
Hence, those skilled in the art have recognized a need for a fluid delivery monitoring system that can detect air in the fluid, but that can also take into account the total air over a period of time or in a volume of fluid, as well as to account for other factors, such as the ability of the patient""s body to safely absorb some air during fluid volume infusion. The present invention satisfies these needs and others.
Briefly and in general terms, the present invention is directed to an apparatus and method for monitoring concentrations of air or other agents, such as undesirable impurities, mixed into a fluid supply system. The invention has particular application in detecting air in a fluid supply system.
The invention includes an agent sensor coupled to a fluid conduit for providing signals in response to agents sensed in the fluid conduit. A processor receives the agent signals from the agent sensor, determining one or more weighted agent signal values by applying a weighting value to one or more of the agent signals based on the volume delivered since each agent signal was received, and determines an agent concentration value from the weighted agent signal values. The processor may compare the agent concentration value to an alarm threshold and, in response to the agent concentration value exceeding the alarm threshold, provide an alarm signal that activates an alarm.
The apparatus may further include a fluid control device, such as a peristaltic pump, acting on a section of the fluid conduit to control the flow of fluid through the fluid conduit, with the processor controlling the fluid control device. In response to the agent concentration value exceeding the alarm threshold, the processor may cause the fluid control device to stop fluid from flowing through the fluid conduit.
The agent sensor may comprise almost any type of sensor capable of detecting agents in a fluid, such as an ultrasonic air detector or an air detector that uses electromagnetic energy, such as light, to detect air in the system. In a preferred embodiment, the agent sensor is an air sensor.
The apparatus may be part of an overall fluid delivery system for introducing fluid to a patient, including a fluid source, a fluid conduit downstream of and in fluid communication with the fluid source, a cannula in fluid communication with the fluid source and configured to be introduced into a patient""s body to provide fluid thereto, an agent sensor coupled to the fluid conduit for providing signals in response to agent sensed in the fluid conduit, and a processor that receives the agent signals from the agent sensor, determines a weighted agent signal value of each agent signal based on the signal and the volume delivered since the signal was received, and processes several weighted agent signal values to determine a primary agent concentration value. The primary agent concentration value is compared to an alarm threshold, and an alarm is activated if the threshold is exceeded.
The agent concentration value may be determined by applying a weighting value to each agent signal as a separate calculation. The weighting value applied to each agent signal value may change based upon the xe2x80x9cagexe2x80x9d of an agent signal. For example, the weighting value may decrease for xe2x80x9colderxe2x80x9d (i.e., less recently received) agent signal values. The xe2x80x9cagexe2x80x9d of an agent signal may be defined as the volume of fluid that has passed since that particular agent signal value was received and/or generated. The xe2x80x9cagexe2x80x9d may also be determined as the actual time that has elapsed since receipt and/or generation of the agent signal value.
The weighting value may take into account numerous parameters. For example, the weighting value may itself be a function of the volume of fluid moved in each sample and the size of the volume window.
The agent concentration value may also be determined by applying a weighting factor to a past agent concentration value, thereby applying that weighting factor to older agent signal values. In such an embodiment, the older agent signal values will effectively have the weighting factor applied to them more often than more recent signal values. If the weighting factor is less than 1, these repeated applications of the weighting factor will cause older signal values to have decreased impact on the agent concentration value.
The invention may further include providing, over a period of time and/or during infusion of a volume of fluid, a second series of agent signal values, and determining a secondary agent concentration value. In one embodiment, the secondary agent concentration value may include weighting values, which may be the same or different from the weighting values used to determine the primary agent concentration value. Alternatively, the secondary agent concentration value may use no weighting values. The secondary agent concentration value can be compared to a secondary threshold, which may be a single bubble threshold, and an alarm may be activated in response to the secondary threshold being exceeded by the secondary agent concentration value.